Hi all, I have a Trane furnance that clicks a lot. Here is the problem. The thermometer calls for heat the inducer fan kicks on, after a while the igniter metal turn bright red, then the 4 burner fires. Moments later the main blower starts. Everything went normal as it should. The problem is after about 2 minutes it start to click randonly and coutinously for about 10 second then the 4 burner shuts off and clicking stops as well. After about 20 seconds, the igniter metal glows bright red and the 4 burner fires. And this on/off cycle continues until the set temperature has been reach. I short the limit switch and burner will continue to burn and runs without issue well over 5 minutes until I stopped it. Because its dangerous to run the system with the limit switch shorted. So my question is what cause the furance to overheat?. I put in a new limit switch, thinking its bad. But the problem continues. I take off the front cover/panel to look at the A-coil. Its clean I can shine a flashlight though it from the outside of the coil to the inside, I can even see the shadow of the fin on the palm of my hand on the inside of the coil. I turn the main blower motor and it turns freely. The air coming out of the vents are pretty strong. So what else could
cause the limit switch to kick in and what cause the clicking on the gas valve area. Any advice appreciated.

You have done a good job so far with a curious problem. The first real data you need is to get the actual temperature rise across the heat exchanger. We need to know exactly how much the temperature is increasing and if that increase is not within the furnace specifications. All indications are it is out of spec but you need the hard data, not symptomatic responses from a sensor. Measure the air temp at the return register (air in) then the temp of the air out at the register closest to the furnace.

The curious part is the clicking you hear. I assume it's the gas valve being closed then open due to some input, possibly the limit switch. Did you check to see if there are 2 limit switches? Normally on an overheat, the gas valve closes once while the blower continues cooling the heat exchanger. Then when the temperature is back to normal, the the ignition sequence starts and the gas valve opens.

If there is an abnormal temperature rise then look to see if the blower motor may be sick. Assuming the blower wheel is clean, a sick motor will cause less than necessary air flow. You might try setting the FAN to ON the call for heat and see if the same situation (clicking) happens. The FAN-ON setting (usually also the AC setting) is usually high speed. Check how your motor is wired to determine what speed HEAT and AC are set.

Lastly remove the control board and check for any cold solder joints or loose connections.